Causes and Rates of Mortality of Swift Foxes in Western Kansas

Discussion

The mortality rates (0.55 for 11-month, 0.57 annual) we observed were similar
to rates observed in other studies of swift foxes. Rongstad et al. (1989. Ecology
of swift fox on the Pinon Canyon Maneuver Site, Colorado. Unpublished final
report to the U.S. Army, Fort Carson, Colorado, USA) reported an annual mortality
rate of 0.48 for a swift fox population on the Pinyon Canyon Maneuver Site in
Colorado in 1986-87, and Covell (1992) reported an annual mortality rate of
0.47 for swift foxes in the same area in 1989-91. Similar to our study, Covell
(1992) found higher survival in fall and lower survival in spring and summer.

Reported mortality rates of kit foxes (Vulpes macrotis), a species
most similar to swift fox, also resemble our results. Ralls and White (1995)
reported an annual mortality rate of 0.42 for a kit fox population on the
Carrizo Plain Natural Area in California. Cypher and Scrivner (1992) reported
annual estimates of 0.53 (1980-84) and 0.68 (1985-90) for kit foxes on the
Naval Petroleum Reserves in California. Disney and Spiegel (1992) reported
annual mortality rates of 0.60 (1990) and 0.32 (1991) for kit foxes in Kern
County, California.

Predation, specifically by coyotes, was the main cause of swift fox mortality.
Covell (1992) reported a similar finding in southern Colorado, where coyotes
were responsible for 85% of all predation on swift foxes. Laurion (1988) also
reported significant swift fox mortality attributed to coyotes. Coyotes have
been reported as the major source of mortality among swift foxes reintroduced
in southern Canada (Carbyn et al. 1994). Kit foxes also are preyed upon heavily
by coyotes (O'Farrell 1987, Ralls and White 1995). Our observation that depredation
of swift foxes always occurred away from dens and core activity areas suggests
this species is more vulnerable to predation in peripheral areas of their
home range.

Poisoning of swift foxes in this study was a localized incident. Information
gathered by state authorities suggested the poisoning was an intentional act,
but the target species was not believed to be swift fox. Primary or secondary
poisoning may affect local swift fox populations, especially where rodenticides
are used to control prairie dogs (Miller et. al. 1994). Poisoning also has
been reported to cause mortality in kit fox populations (Schitoskey 1975,
Standley et al. 1992). There is no evidence of widespread poisoning risks
to swift fox populations, although predator control programs that included
poisoning apparently contributed to the decline in swift fox numbers in the
late 1800s and early 1900s (Johnson and Sargeant 1977, Zumbaugh and Choate
1985). If we remove poisoned foxes from our calculations, the mortality rate
for adult swift foxes was 0.46 ± 0.08 (
± SE) overall and 0.47 ± 0.12 in the Rangeland Area.

Swift foxes potentially have high reproductive rates, which may compensate
for high mortality. We estimated litter size to average 3.25 ± 0.34, using
observations of 8 dens at which we could count pups as they emerged (M. A.
Sovada and C. C. Roy, unpublished data). Average litter sizes for swift foxes
range from 3.4 to 5.7 (Kilgore 1969; Hillman and Sharps 1978; Rongstad et
al. 1989. Ecology of swift fox on the Pinon Canyon Maneuver Site, Colorado.
Unpublished final report to the U.S. Army, Fort Carson, Colorado, USA; Covell
1992; Carbyn et al. 1994).

Mortality rates of juveniles reported in this study could be misleading
because of small sample sizes and lack of independence among sampled animals
(i.e., sibling relationships), although radiocollared siblings never died
together. The difference in juvenile mortality due to vehicle collision in
Cropland and Rangeland areas may be attributed to 90% more roads in the Cropland
Area. Moreover, juveniles are probably less wary than adults and may have
been at greater risk to vehicle collision (Storm et al. 1976). Whether vehicle
collisions are additive or a compensatory mortality factor for swift foxes
is unknown.

Although highly cultivated landscapes are not considered suitable to support
sustainable populations of swift foxes (Samuel and Nelson 1982), a few studies
have indicated swift foxes inhabit areas of mixed agricultural use (Kilgore
1969, Hines 1980). Our results show no difference in mortality rates between
swift foxes in the Cropland Area and Rangeland Area.

Changes in the canid community within the historic swift fox range seem
to have been detrimental to swift fox populations. Swift foxes apparently
thrived in the region when the canid community was dominated by gray wolves
(Johnson and Sargeant 1977). Gray wolves, unlike coyotes, apparently paid
little attention to the smaller swift fox, and chances of swift fox encounters
with wolves probably were less than with coyotes, because of the larger home
range size and lower overall density of wolves compared to coyotes (Johnson
and Sargeant 1977). More importantly, wolves likely kept the numbers of coyotes
significantly depressed (Johnson and Sargeant 1977, Sargeant et al. 1987).
There is considerable evidence that interspecific competition, often as interference
competition, acts as a mechanism regulating spatial distribution and population
size among canid species (Carbyn 1982, Rudzinski et al. 1982, Sargeant et
al. 1987, Bailey 1992, Ralls and White 1995). Scott-Brown et al. (1987) speculated
that the decline of the swift fox may be a direct result of competition with
and predation by coyotes.